As a model system for studying the effects of subunit interaction on enzyme catalysis, two thiokinases, succinate thiokinase and malate thiokinase will be compared. Both enzymes are found in the bacterium Pseudomonas MA grown on methylamine. The two thiokinases catalyze the identical reaction, have similar subunit structure, and have the same reaction intermediates. These two enzymes will be compared a) in terms of their kinetic properties with respect to half-site reactivity, b) in terms of sequence homology with respect to active site peptides and c) in terms of sequence homology with respect to the individual polypeptide chains comprising the native enzyme. In addition, the function of the individual polypeptide chains of each of the enzymes will be examined by studying the catalytic properties of isolated subunits and mixed hybrids formed between the two enzymes. These studies should help elucidate the role of individual subunits in enzyme catalysis and the effects of subunit interactions on catalysis.